The present invention relates to a process for recovering at least a portion of the boron trifluoride (BF3) present in a catalytic mixture comprising BF3/alcohol catalyst complex used as a promoter in an oligomerization process for alpha olefins.
Boron trifluoride is the catalyst of choice for producing polyalphaolefins (PAO) of varying viscosity grades. See for example U.S. Pat. Nos. 4,956,512 and 5,945,574. Boron trifluoride is a gas which is usually combined with a protic promoter, typically an alcohol such as butanol, to form a catalyst complex which may be used to promote the oligomerization of alpha olefins, such as for example 1-decene or 1-dodecene. The boron trifluoride represents a significant cost in process schemes for producing polyalphaolefins. Furthermore, since used catalyst is often disposed of by deep-well injection in commercial operations producing polyalphaolefins, the disposal of the used catalyst has some environmental considerations.
Various methods for recovering the boron trifluoride have been proposed. See for example U.S. Pat. No. 2,167,358 which uses a trivalent nitrogen compound to form a complex with the boron trifluoride. U.S. Pat. No. 5,846,429 teaches the adsorption of the boron trifluoride on a synthetic polymer containing a nitrile group and then recovering the boron trifluoride by heating the polymer fibers. Other methods for recovering the boron trifluoride include thermally cracking the boron trifluoride complex (European Patent Application 96303173.7) and recovering the boron trifluoride as a hydrate (European Patent Application 89118395.6). U.S. Pat. No. 5,254,784 describes a method for recovering excess boron trifluoride from the boron trifluoride/alcohol complex without decomposition of the complex by distilling the reaction product under reduced pressure, recovering the excess boron trifluoride as an exhaust gas, forming additional complex by using alcohol as the liquid ring or fluid moment in either a liquid pump or a torque pump, and recycling the complex to the reactor.
Unfortunately none of the processes described in the prior art have been proven to be practical in commercial operations for the oligomerization of alpha olefins. The present invention is directed to a practical method for recovering boron trifluoride from a product stream, combining the recovered boron triflouride with a protic promoter, and introducing the fresh mixture into the reactor. Pilot plant studies have demonstrated that the process scheme of the present invention significantly increases the efficiency of the process.
In its broadest aspect, the present invention is directed to a method for separating at least part of the boron trifluoride present in a mixture comprised of boron trifluoride and an alcohol having from 1 to about 10 carbon atoms, wherein excess boron trifluoride is present in the mixture than used to form a complex of boron trifluoride and alcohol, the method comprising (a) reducing the pressure of the mixture sufficiently to cause the separation of boron trifluoride vapor from a liquid organic phase which Includes boron trifluoride/alcohol complex; (b) recovering the boron trifluoride vapor separately from the liquid organic phase; and (c) mixing the boron trifluoride vapor from step (b) with fresh alcohol using countercurrent flow to form additional boron trifluoride/alcohol complex. This method is particularly useful in a process which utilizes the boron trifluoride/alcohol complex as a catalyst in a reaction zone for the oligomerization of alpha olefins.
The present invention is further directed to a process scheme for the oligomerization of an apha-olefin in the presence of boron trifluoride catalyst which comprises (a) contacting in an oligomerization zone an alpha olefin feed with a catalytic mixture comprising an effective catalytic amount of a catalyst complex consisting of boron trifluoride and a protic promoter under conditions and for a time sufficient to oligomerize the alpha olefins in the feed whereby a liquid organic mixture is formed comprising polyalphaolefins and the catalytic mixture; (b) recovering the liquid organic mixture from the oligomerization zone; (c) reducing the pressure of the liquid organic mixture recovered in step (b) sufficiently to separately recover a gas comprising at least a portion of the boron trifluoride present in the catalytic mixture and a liquid product stream comprising the polyalphaolefin product and residual catalyst complex; (d) mixing the gaseous boron trifluoride recovered from step (c) with fresh protic promoter to form fresh catalyst and promoter mixture; (e) introducing the fresh catalyst complex from step (d) into the oligomerization zone of step (a); and (f) recovering the polyalphaolefin product from the liquid product stream of step (c). In a preferred embodiment of the process scheme, the residual catalyst complex is also recovered separately from the polyalphaolefin product in step (f) above and is recycled back to the oligomerization zone of step (a).
The protic promoter forms a catalyst complex with the boron trifluoride, and the complex serves as a catalyst for the oligomerization of alpha olefins. The protic promoter will usually be an alcohol having from 1 to about 10 carbon atoms in the molecule. Preferably the protic promoter is n-butanol. The catalytic mixture comprising the boron trifluoride/alcohol complex usually also contains excess boron trifluoride which is adsorbed in the mixture. It is primarily the excess boron trifluoride which is recovered by the method described herein.
The alpha olefin feed will generally comprise an alpha olefin or a mixture of alpha olefins having from about 6 to about 20 carbon atoms in the molecule. In one preferred embodiment of the invention, the alpha olefin feed will comprise decene or a mixture of alpha olefins containing decene. In carrying out the process scheme when n-butanol is the protic promoter, the pressure in the oligomerization zone is preferably maintained at 40 psig or greater. In recovering the boron trifluoride from the catalytic mixture, the pressure is preferably reduced to at least 2.0 psia or lower and more preferably will be reduced to 0.1 psia or less. However one skilled in the art will recognize that the optimal pressure in the oligomerization zone and also used for the recovery of the boron trifluoride may vary somewhat depending on the protic promoter employed. However, finding the optimal conditions for carrying out the process with the various operable protic promoters should involve only routine testing.
A particular advantage of the present invention is that a much smaller absorbing vessel for the preparation of the catalyst complex may be used than would otherwise be necessary when a conventional Schott reactor is employed. The smaller absorbing vessel makes it possible to accelerate the time required to reach a steady state in the vapor absorber. The process of the present invention also significantly increases the efficiency of the boron trifluoride catalyst.
As used in this disclosure the phrase xe2x80x9ceffective catalytic amountxe2x80x9d refers the amount of catalyst necessary to promote the polymerization of the alpha olefin or mixture of alpha olefins present in the feed.
As used in this disclosure the words xe2x80x9ccomprisesxe2x80x9d or xe2x80x9ccomprisingxe2x80x9d is intended as an open-ended transition meaning the inclusion of the named elements, but not necessarily excluding other unnamed elements. The phrase xe2x80x9cconsists essentially ofxe2x80x9d or xe2x80x9cconsisting essentially ofxe2x80x9d is intended to mean the exclusion of other elements of any essential significance to the combination. The phrase xe2x80x9cconsisting ofxe2x80x9d is intended as a transition meaning the exclusion of all but the recited elements with the exception of only minor traces of impurities.